High velocity ignition-propagating cord



y 1967 w. E. SCHULZ HIGH VELOCITY IGNITION-PROPAGATING CORD Filed Sept. 1, 1964 INVENTOR WILLIAM E. SCHULZ ATTORNEY United States Patent 3,320,882 HIGH VELOCITY IGNITION-PROPAGATING CORD William E. Schulz, Wenonah, N..I., assignor to E. I. du

Pont de Nemours and Company, Wilmington, Del., a

corporation of Delaware Filed Sept. 1, 1964, Ser. No. 393,561 8 Claims. (Cl. 102-27) This application is a continuation-impart of my copending application Ser. No. 343,994, file-d Feb. 11, 1964, now abandoned.

The present invention relates to a novel explosive cord by which an ignition stimulus is propogated at an extremely high velocity.

For many years, propellant and rocket ignition engineers have recognized the necessity for a rapid, lightweight, and uniform ignition system to insure optimum performance of the propellant charge and motor systems. Although major contributions have been made toward this goal, e.g., by the provision of igniter cords such as described in US. Patents Nos. 2,974,596 and 3,062,147, assigned to the present assignee, a need still exists for an ignition-propagating cord which is safe to handle and store, which will reliably bring about the desired rapid ignition of the propellant compositions with a minimum of brisance and violence to the components of the assembly in which it is to be used, and preferably, is flame sensitive.

Accordingly, an object of this invention is to provide a high velocity, ignition-propagating cord having low brisant detonation. Another object of this invention is to provide an ignition-propagating cord that effects ignition of the entire exposed surface of the propellant grain or charge substantially simultaneously with extreme rapidity. A further object of this invention is to provide an ignition-propagating cord that is flame sensitive and capable of being initiated from a hot flame or spark and yet is insensitive to ordinary shock and stray electrical currents. Still another object of this invention is to provide an ignition-propagating cord that is particularly suited for and effective for use with combustable cartridge ammunition. Another object of this invention is to provide an ignition-propagating cord that can impart to normally slow burning conventional ignition materials a much faster propagating rate, and thus ignite the propellant more rapidly. Another object of the invention is to provide an ignition-propagating cord that is relatively inexpensive tomanufacture and especially suited for use with propellant charges and more particularly for use to ignite pyrotechnic compositions used to ignite a propellant grain.

In accordance with this invention such an ignitionpropagating cord is now provided which comprises a continuous column of a mixture comprising, by weight, about 50 to about 95% of a high explosive compound and about 50 to about 5% of particulate metal confined in a metal sheath and the distribution of the mixture being at least about 0.5 grain per foot of sheath length, the high explosive compound being present in an amount of at least about 0.25 grain per foot of sheath length.

In order to describe the invention in greater detail, reference now is made to the accompanying drawings, in which,

FIGURE 1 is a sectional view of the ignition-propagating cord of this invention; and

FIGURES 26 inclusive are sectional views of typical assemblies incorporating the ignition-propagating cord of this invention, these views being given to illustrate representative uses of the cord.

In FIGURE 1, 1 represents a ductile metal sheath, preferably of lead or a lead alloy, and 2 represents the mixture of high-explosive compound and particulate metal contained within the sheath.

In FIGURE 2, 3 represents a casing which is shown as tubular in configuration and has one end closed and the other forming a nozzle 3A. Within casing 3 is a customary charge or elongated propellant grain 4 having a central cavity throughout its entire length. These components of a rocket motor are conventional. Disposed axially within the cavity of the grain 4 and extending substantially the length of the charge, is the ignitionpropagat-ing cord 5 of the present invention. In the embodiment shown the cord is maintained in position within the cavity by washers 6 so that an air space 7 is provided between the cord 5 and the grain 4. In initiating relationship with one end of the cord is an electric initiator 8 having lead wires, 9, i.e., electric conductors, extending to a source of electric current (not shown).

In FIGURE 3, the elements are substantially the same as those in FIGURE 2, however in this embodiment the ignition cord 5 is helically coiled on a mandrel 10 which is maintained in spaced relationship from the grain 4. In this embodiment, in order to obtain a high degree of reliability, an electric or nonelectric end initiator is used to initiate both ends of the cord. Notethat a spacing is maintained between each turn of the cord in coil or winding. This spacing is requisite to avoid cut-offs due to cross-propagations between coils.

Direct ignition of the grain may be obtained by threading the ignition-propagating cord through longitudinal bores of the propellant grain 4 as shown in FIGURE 4, and several separate grains can be strung in equally spaced relationship along the same length of cord. If desired, a flexible elongated rope or cable of suitable material such as of nylon, sisal, hemp, etc., may extend through a bore parallel to and in close proximity to the bore for the cord, provided that the provision of this rope or cable has no detrimental effects on the burning characteristics of the grain.

Similarly, direct or semi-direct ignition of the grain can be obtained by incorporating coils or loops of the ignition-propagating cord into the surface of the grain or merely having the cord in the immediate proximity of the coating, or into an igniter coating 12 covering the surface of the grain as shown in FIGURE 5.

In FIGURE 6, an artillery cartridge assembly is shown wherein the ignition-propagating cord of this invention is utilized to initiate a cylindrical (tubular) mass of a propellant composition. As may be seen in this figure, the ignition-propagating cord 5 extends through a longitudinal bore of propellant charge 4, which may be of a composition such as smokeless powder, the cord being in contact with an igniter coating 12 covering the surface of the bore. As is the case in FIGURE 5, the igniter coating 12 is of a composition which is more easily ignited than is the mass of propellant charge.

The cord may also be used in conjunction with jelly roll or basket-type igniters. It is to be understood that the above visualized applications are intended only to show the versatility of the cord of this invention and that these applications are but a few of the wide variety of uses for the cord combining rapid ignition and low brisant detonation. For example, the cord may be used to ignite gas generator propellants for auxiliary power units, artillery gun charges, mortar charges, cartridge activated devices, oil well perforating guns, cannons, smoke bombs, etc., or may be used to ignite an igniter coating or column in the main propellant grain.

The functioning of the ignition-propagating cord is similar in each of these applications. Actuation of the cord by electric or nonelectric initiator 8, e.g., a flame or spark, causes ultrarapid transmission, e.g., at about 1800-6000 meters/ second, of the ignition stimulus down the length of the grain or charge adjacent to the cord so that substantially simultaneous deflagration of the entire internal surface of the grain occurs. This simultaneous deflagration results in an immediate actuation of the grain. The ignition-propagating cord of this invention gives excellent results with respect to its igniting ability and the velocity at which the ignition stimulus is transmitted. The desired velocity, i.e., 1800' or more meters per second, is obtained when the cord contains at least about 0.25 grain of high explosive per foot of length. By varying the composition and distribution of the mixture of high explosive and particulate metal within the metal sheath not only the velocity of transmittal of the ignition but also its igniting ability can be controlled. When the cord contains from at least about 0.25 grain of high explosive per foot of length, the distribution of the mixture can be varied upward from about 0.5 grain per foot without deleterious effects upon either the velocity or igniting ability of the cord. For most applications, the core loading (distribution) of the mixture in the cord will not exceed about 50 grains per foot of sheath length, and preferably where nonviolent, nonbrisant ignition is particularly desired, the core loading will not exceed about 20 grains per foot and, generally, will not be less than about 1 grain per foot. However larger core loadings may be used but the core loading will not exceed about 200 grains per foot. The use of less than about 0.5 grain of mixture per foot is generally not feasible, since the energy of cords containing the mixture at such low distribution for most purposes does not consistently insure reliable ignition of the propellant grain or charge.

The proportion of the components of the mixture of the ignition-propagating cord can be varied within wide ranges without deleterious effects. The high explosive component should comprise about 50-95% of the mixture, the particulate metal comprising 50 to 5%. The ignition effect of cords containing the higher percentages of particulate metal is somewhat less than that containing lower percentages, e.g., to 25%, owing to the metal particles acting as a heat sink, thus absorbing energy which could be used to ignite the grain. Optimum results have been obtained with a mixture of about 60% lead azide and 40% particulate metal, e.g., aluminum, when the cord is used to ignite completely the combustible artillery ammunition, smoke bombs and the like. By the proper selection of the proportions of these components, a wide variety of propellant charges can be ignited by the ignition-propagation cord at various velocities. For example, when ignition at high velocity of an easily ignited propellant is required, a cord containing a high percentage of high explosive and low percentage of particulate metal is used. For more difficultly ignited propellants, a cord containing a higher proportion of particulate metal and correspondingly decreased amount of high explosive is used. Since, at a given distribution of the mixture within the cord, the lower the high explosive content is the lower will be the velocity of transmission of the ignition stimulus, cords containing the lower proportions of high explosive will transmit the ignition stimulus at lower velocities. By increasing the distribution, i.e., the core loading, of the mixture, or the total quantity of mixture per unit length, the velocity will be increased.

Representative of crystalline high explosives which can be used in the ignition-propagating cords of this invention are azides, such as lead azide, picryl sulfone; organic nitrates such as PETN and nitromannite, nitramines such as cyclotrimethylenetrinitramine (RDX), cyclotetramethylenetetranitramine (HMX), and tetryl; nitro compounds such as TNT, and the tetranitro compound of dibenzo- 1,3a,4,6a-tetrazapentalene (Tacot). Mixtures of these high explosives also are suitable. The explosive may be of very fine granulation or standard grades, for example, cap grade, can be used.

Explosive compositions, such as lead azide, which are flame-sensitive and low-brisant, are preferred for use in many applications, particularly when it is desired to use the cord in conjunction with an igniter coating on the propellant charge. The combination of lead azide and a particulate metal such as aluminum have been found to be unique, and ignition-propagating cords that utilize lead azide as the crystalline high explosives produce unique and quite unexpected results. For example, such ignition-propagating cords containing lead azide are flame sensitive and can be initiated by a flame or a spark. Therefore, this precludes the use of conventional end initiators or boosters with all their attendant disadvantages. In addition, the unique combination of lead azide and particulate metal in the ignition-propagating cords impart to normally slow burning ignition materials that are in ignitable contact with the propellant, for example, igniter mixes in the form of a separate unit or coated on the inner surface of the propellant, e.g., an artillery shell containing smokeless powder, and particularly completely combustible artillery shells, a much faster propagating rate and this, in turn, has the effect of igniting the entire exposed surface of the propellant substantially simultaneously. Therefore, the lead azide-particulate metal ignition cord causes conventional ignition materials, e.g., black powder, compositions containing boron and potassium nitrate, compositions comprising silicon, potassium nitrate, charcoal and iron oxide, and the like, in ignitable contact with the surface of the propellant to ignite at the speed of the ignition cord itself, for example, velocities of the order of 1800* to 3000 meters per second. Due to the superior ignition properties of the unique combination of lead azide and particulate metal, the brisance of the ignition cord is kept at a minimum.

The ignition cord of lead azide-particulate metal can also contain other crystalline high explosives in admixture with lead azide. For example, representative capsensitive materials such as RDX, HMX, PETN, TNT and the like can be mixed with lead azide. However, in order that the particular beneficial effects of the lead azideparticulate metal are not lost, not more than about 25% of a cap-sensitive high explosive (based on total weight of explosive) can be used with lead azide when a flamesensitive, low-brisant cord is required.

The lead azide used for nonviolent low brisant action if preferably of high purity, e.g., that having a lead azide content of about 98.5% which is commercially known as RD-1333 or R'D-l343 lead azide and more particularly described on pages A557 to A563 of the Encyclopedia of Explosives and Related Items, volume 1, published by Picatinny Arsenal, Dover, N]. This degree of purity is desirable since it makes possible closer control of the composition of the mixture and does not introduce extraneous diluents which might have a deleterious effect on the propagation characteristics of the cord. However, dextrinated lead azide. may also be used.

As the particulate metal, various metal powders including aluminum, magnesium, boron, titanium, zirconium, tellurium, selenium, and cadmium as well as mixtures thereof can be used. The particle size of the met-a1 is not critical and may range from finely divided powders about 1 micron in size to relatively larger particles about micron in size. Suitable metal shapes which may be utilized are, for example, ground particles, atomized pellets, band saw dust, flakes, metal shavings, rotary fillings, and the like. It has been found that rivet grade aluminum powder is particularly suitable for use with high purity lead azide. This particulate aluminum has a loose density of 0.76 to 0.88 gm./cc. and a particle size that 100% passes through a 48 mesh standard Tyler screen, -20% is held on an 80 mesh screen, 30 40% is held on a 100 mesh screen and 2% passes a 200 mesh screen. However, other particulate, comminuted forms of aluminum, e.g., grained aluminum, atomized aluminum, and flake-like aluminum powder, can also be used. The particulate metal can be a mixture of two or more grades of comminuted metal.

The ignition propagation cords of this invention are prepared by filling a tubular sheath which preferably is of a relatively heavy-walled ductile metal, such as lead, a lead alloy, aluminum, or the like with a certain quantity of a mixture of the high explosive and particulate metal, preferably, but not necessarily, a grained mixture, and subsequently. drawing, swaging, or rollingthe tube until substantially the desired distribution of the mixture is obtained. Of course, the ductile metal selected should not be reactive with any component of the mixture under predicted conditions of storage and handling of the cord. Inasmuch as one of the most desirable properties of a propellant igniter is a minimum of residue, lead is highly desirable as the sheath material since the hot metal particles which are produced by action of the explosive particles are very small and vaporize to a great extent such that only a small amount of residue and trash remain. The small metal particles produced by actuation of the cord actually scour and clean the propellant surface and allow the cord to ignite wet, dirty, or otherwise inhibited propellant grains.

The weight of the ductile metal sheath used will vary. Naturally, at low loadings and with low proportions of high explosive compound, a thinner sheath will be used than at higher loadings. In general, the thickness of the sheathing will be about equal to the diameter of the core. For example, the weight of lead sheath will be about 3 grams per foot of length at a core loading of 1.0 grain per foot thus insuring complete fragmenting of thi sheath and reliable initiation of the propellant charge.

When the ignition-propagating cord is to be used in an atmosphere which contains a high percentage of nitrogen or an inert gas it is desirable to use a propellant composition having a positive oxygen balance or to contain the cord in a chamber having an oxygen rich atmosphere. Under most conditions of use in the earth atmosphere however, the provision of a supplemental supply of oxygen is not necessary.

The following examples serve to illustrate specific embodiments of the ignition-propagation cord of this invention. However, they will be understood to be illustrative only and not as limiting the invention in any manner.

Example 1 A mixture comprising 60% of high-purity lead azide and 40% rivet grade aluminum was blended in a Fisher- Kendall blender and the blend mixed with a small quantity of a 1% solution of pyronitrocellulose in butyl acetate (50 g. mixture/l0 cc. of solution) to the extent that the powder was substantially completely wetted by the solution. The paste so formed was subsequently pushed through a 42 Tyler mesh bolting cloth screen and the screened mixture dried 8 hours at 120 F. The dried powder was cooled, then passed through a 28 mesh screen to break up lumps. The mixture, which had a loading density of about 1.1 g./cc., subsequently wa vibrated into a lead tube 19 inches long and 0.230 inch in inner diameter and 0.500 inch in outer diameter. The weight of the mixture loaded was 13.2 grams. The tubing was drawn out by means of a series of dies into a tube 0.055 inch in outer diameter. The distribution of the mixture in the tube was 2.0 grains/ft. The ignition-propagating cord thus prepared was tested by positioning an 8-inch length of the cord along the longitudinal axis of a cardboard tube inch in inner diameter and 1 inch long and filling the annulus around the cord with loose black powder. The cord was initiated at one end by an electric squib. Upon actuation, the cord detonated at a velocity of 2522 meters per second and ignited, substantially simultaneously, the entire charge of the propellant powder.

An unused portion of the cord prepared above was drawn to an outer diameter of 0.050 inch, and a load distribution of 1.6 gr./tt. When a portion of this cord was tested as described above, the cord detonated at 2558 meters per second and effected substantially simultaneous ignition of the propellant powder.

A second portion of the cord was drawn to an outer diameter of 0.045 inch and a load distribution of 1.3 gr./ ft. When tested, this cord detonated at a velocity of 2230 meters per second and ignited the propellant powder.

Example 2 7V 7 An ignition-propagating cord was made from a mixture comprising 60% lead azide and 40% aluminum prepared as described in Example 1, placed into a lead tube 0.312 inch in diameter and 60 inches long, swaging the tube to an outer diameter of 0.073 inch, then drawing to a load distribution as shown in the table below. The cord was initiated by an electric initiator containing a base load of 1.5 grains of lead azide. Velocity tests of the cord gave The procedure of Example 1 was again followed in preparing an initiation-propagating cord having an outer diameter of 0.055 inch and containing an /20 mixture of high purity lead azide/ rivet grade aluminum at a distribution of 2.5 gr./ft. This cord was strung through a smoke bomb having a central core lined with an igniter mix such that the igniter mix was adjacent to the cord. The igniter mix was a composition prepared from a mixture of 26% silicon, 35% potassium nitrate, 4% charcoal, 22% black iron oxide and 13% aluminum grained with a 96/4 acetone/ nitrocellulose solution. The cord detonated at 3618 meters/second and the smoke bomb was ignited substantially simultaneously.

Example 4 A series of mixtures comprising cap grade PETN and metals in proportions as shown in Table 2 below, was prepared by a procedure similar to that used in preparing the mixture of Example 1. These mixtures were vibrated into lead tubes and the diameters of the tubes were reduced by drawing by a procedure substantially as described in Example 1. The cords, their core compositions and loadings and their velocity of detonation are summarized in the following table. The ignitive effect of the cords was ascertained by using a photoelectric cell, shock-mounted by means of sponge rubber, in one end of a 3-inch diameter steel pipe. The five-foot-long pipe was fitted with a square chamber 18 inches from the end holding the photocell. The chamber was open at one edge and held a safety glass and metal insert containing a centrally located /2-inch iris for passage of light. At the opposite end of the pipe was a removable cap containing a small central hole through which the ignitionpropagation cord was inserted. The cord was initiated on the outside of the pipe by means of an electric initiator. The output of the photocell was fed to the vertical axis of an oscilloscope via a power amplifier. Oscillograms recorded the light flashes caused by detonation of a 1-inch length of the cord. The peak intensities were measured in volts and the duration of the light in milliseconds. The intensity of the light was a measure of the flame temperature. Each of these cords could transmit an ignition impulse to an adjacent propellant compound.

TABLE 2 Average Light Velocity Output Core of Deto Core Composition Loading, nation,

gr./it. JIL/SGO Dura- V. ms. tion 50/25/25 PETN/Al /A1 2 19 20-40 94/6 PETN/Al 5. 4 5, 860 l 1 75/25 PEIN/Al 2 4. 8 5, 650 5 11 50/50 PE'IN/Al 2 6. 4 5, 260 31 35-40 75/25 PETN/Al 5. 1 5, 050 7 8-12 50/50 PETN/Al l 7.2 6, 350 20 25-35 50/25/25 PETN/Al /A1 7. 4, 020 21 25-35 50/50 PETN/Mg c 7.6 5,080 14 30 50/50 PETN/Al coarse. 4. 4 4, 620 6 7-8 1 Aluminum 101, particle size, 100 mesh (Tyler standard screen). 2 Rivet grade aluminum.

Similar results are obtained When RDX or HMX is used as the explosive compound in mixtures prepared as described above.

The ignition-propagating cord is insensitive to ordinary shock and stray electric currents and difficult to initiate from the side. The cord is actuated by applying to one end a conventional initiator such as an electric squib. In cases where lead azide is used as the high explosive compound, a beaded bridgewire arrangement, an exploding bridgewire, or an arc-firing system may be used to actuate the cord. Advantageously, cords containing lead azide as the explosive compound also may be ignited readily by the flame of, for example, a conventional percussion primer. Cords having a core composed of lead azide and aluminum are especially preferred particularly for use in conjunction with a coating or column of an igniter composition or mix which is more easily ignited, i.e., more flame-sensitive, or more readily ignitable, than is the main propellant charge. In these cases, the cord imparts to the normally slow burning igniter composition a faster burning rate thus enabling this composition to ignite the propellant more rapidly. This arrangement is especially useful in igniting artillery cartridges having a combustible case, propellant grains, chemical grenades, and clusters of chemical sub-munitions.

I claim:

1. A high velocity ignition-propagating cord consisting of, by weight, a continuous column of a mixture of from about 50 to 95 percent lead azide and from about to 50 percent particulate metal confined in a metal sheath at a distribution of from about 0.5 to 200 grains of said mixture per foot of length of said sheath, said lead azide being present in an amount of at least about 0.25 grain per foot of length of said sheath. 1

2. A high velocity ignition-propagation cord consisting of, by weight, a continuous column of a mixture of from about 50 to 95 percent lead azide and from about 5 to 50 percent particulate metal confined in a metal sheath at a distribution of from about 1 to 20 grains of said mixture per foot of length of said sheath, said lead azide being present in an amount of at least about 0.5 grain per foot of length of said sheath.

3. The ignition-propagating cord of claim 2 wherein the particulate metal is aluminum.

4. The ignition-propagating cord of claim 2 wherein the metal sheath is lead.

5. A high velocity ignition-propagating cord consisting of, by weight, a continuous column of a mixture of from about 50 to percent of crystalline high explosive comprising lead azide and from about 5 to 50 percent of particulate metal confined in a metal sheath at a distribution of from about 0.5 to 20 grains of said mixture per foot of length of said sheath, said high explosive compound being present in an amount of at least about 0.25 grain per foot of length of said sheath.

6. The ignition-propagating cord of claim 5 wherein the lead azide is dextrinated lead azide.

7. A high velocity ignition-propagating cord consisting of, by Weight, a continuous column of a mixture of from about 50 to 95 percent of crystalline high explosive comprising lead azide and up to about 25 percent based on the total weight of explosive of PETN and from about 5 to 50 percent of particulate metal confined in a metal sheath at a distribution of from about 1 to 20 grains of said mixture per foot of length of said sheath, said high explosive compounds being present in an amount of at least about 0.5 grain per foot of length of said sheath.

8. A high velocity ignition-propagating cord consisting of, by Weight, a continuous column of a mixture of about References Cited by the Examiner UNITED STATES PATENTS 2,959,001 11/1960 Porter 1027O X 2,974,596 3/1961 Allen 10270 3,147,710 9/1964 Gluckstein 10270 3,156,186 11/1964 Picciano et al. 102-28 3,207,073 9/1965 Miller 102-27 3,264,150 8/1966 Leslie 14935 References Cited by the Applicant UNITED STATES PATENTS 5/1961 Andrew et al. 9/1963 Ciccone et al. 

1. A HIGH VELOCITY IGNITION-PROPAGATING CORD CONSISTING OF, BY WEIGHT, A CONTINUOUS COLUMN OF A MIXTURE OF FROM ABOUT 50 TO 95 PERCENT LEAD AZIDE AND FROM ABOUT 5 TO 50 PERCENT PARTICULATE METAL CONFINED IN A METAL SHEATH AT A DISTRIBUTION OF FROM ABOUT 0.5 TO 200 GRAINS OF SAID MIXTURE PER FOOT OF LENGHT OF SAID SHEATH, SAID LEAD AZIDE BEING PRESENT IN AN AMOUNT OF AT LEAST ABOUT 0.25 GRAIN PER FOOT OF LENGTH OF SAID SHEATH. 